Check out out series on "Finding and Understanding Sources", which should hopefully prove to be helpful for you as you continue in your research.

Anyways though, the Spanish Civil War is a rather broad topic, and one on which these are thousands of books on. What aspect of the war are you interested in? The Nationalists, the Republic, the Anarachists, the Internationals? Political, Social, Military angle? No one here can tell you what topic to write on, you need to find one you connect with. But helping narrow down what you are looking for can help us at least recommend you sources to continue your own research with.

As mentioned above, it's a fairly broad topic. If you could narrow it down, I could recommend some anarchist sources on a few topics.